Films suitable for consumer products, such as bags and packages, are commonly produced from oriented polypropylene (OPP) films. Such films for consumer packaging applications are typically multi-layered having a core layer and one or more coating layers disposed thereon to improve blocking and slip characteristics of the film.
The blocking and slip characteristics of the film are very important, especially when the film is processed on high-speed packaging equipment. A film that exhibits good blocking and slip characteristics typically provides good processability on high-speed packaging equipment. The sealing characteristics of the film is also very important. In hot seal packaging applications, the seal can be formed by heat and pressure. In cold seal operations, the seal can be formed by first applying an “adhesive” to the film area to be sealed followed by pressure.
Uncoated thermoplastic films, including OPP films, have poor sealing characteristics. In an effort to address these problems, coatings that provide improved heat-sealing characteristics are being developed. However, there is a tradeoff because coatings that can improve seal characteristics often adversely affect blocking and slip characteristics.
For example, U.S. Pat. No. 5,419,960 discloses low temperature seal coating compositions comprising a copolymer of about 65 to 95 wt % ethylene and about 5 to 35 wt % of acrylic or methacrylic acid, based on the weight of the polymer, in which about 2 to 80% of the carboxylate groups are neutralized with metal ions from Group IA, IIA or IIB of the Periodic Table, preferably sodium ions.
WO 2005/026275 discloses a coating composition that includes an aqueous dispersion of a substantially non-polar copolymer of ethylene and an alpha-olefin having less than 20 carbon atoms, and an additive selected from the group consisting of an anti-blocking and slip agent, a coalescent and wetting agent, and mixtures thereof.
U.S. 2005/0100754 discloses aqueous dispersions comprising at least one thermoplastic resin, at least one dispersing agent, and water, wherein the dispersion has a pH of less than 12, including aqueous dispersions having a volume average particle size of less than about 5 microns (5000 nanometers). The dispersing agents can include ethylene-carboxylic acid polymers, and their salts, such as ethylene-acrylic acid copolymers or ethylene-methacrylic acid copolymers, and others.
WO 2005/085331 discloses aqueous dispersions comprising at least one thermoplastic resin, at least one dispersing agent, and water, wherein the dispersion has a pH of less than 12, including aqueous dispersions having a volume average particle size of less than about 5 microns (5000 nanometers). The thermoplastic resins include alpha-olefin interpolymers of ethylene with at least one comonomer selected from the group consisting of C4-C20 linear, branched or cyclic dienes, or an ethylene-vinyl compound, among others. The dispersing agents include ethylene-carboxylic acid polymers, and their salts, such as ethylene-acrylic acid copolymers or ethylene-methacrylic acid copolymers, and others.
In addition to the coating itself, the technique used to apply the coating to the film can have a drastic effect on the overall film characteristics. Coatings have been typically applied to films using in-line or off-line techniques. In-line coating is done during the film forming process, typically before or after a stretching operation, and before heat-setting the film. Conversely, off-line coating is done after the film has been formed and after the heat-setting stage and is usually not performed during film the manufacturing step, but rather as a separate step. Moreover, off-line coating processes tend to provide films having lower adhesion than in-line coated films. As a result, a primer is typically used for off-line coating processes to improve adhesion. Consequently, off-line coating is less cost effective.
In the case of polyolefin dispersion coatings, the resulting film often exhibits less than desirable haze. For example, WO 2005/026275 discloses off-line coating its polyolefin dispersions to films. However, the resulting films show a higher than desired haze level. The high haze is believed to be due to the particle sizes of the polyolefin in the water based dispersion being too large. In fact, during the manufacturing process on making the dispersion, it is very difficult to keep the polyolefins dispersed in very small particle sizes.
Furthermore, off-line coating applications may use organic solvents and cosolvent, which are not suitable for in-line coating. Organic solvents typically cause sticking or blocking of the film during the winding operations used during film manufacture and may be harmful, hazardous in use, or toxic and detrimental to the environment. Besides the problems of disposal of such hazardous, or toxic materials, films made in this way often contain a residual amount of the solvent, and so may not be suitable for use in applications where they come into contact with food products. U.S. Pat. Nos. 6,939,584; 5,525,421; 4,956,241; and 4,956,233 discuss in-line or off-line coating techniques.
There is a need, therefore, for cost effective methods for making films, the films made therefrom that exhibit excellent block, slip and sealing characteristics.